So-called Liar Flyer is a fine-feathered mimic (8)
I believe the answer is:
lyrebird
'mimic' is the definition.
(I've seen this before)
'so-called liar flyer' is the wordplay.
'so-called' indicates a 'sounds like' (homophone) clue.
'flyer' becomes 'bird' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'liar' is a homophone of 'lyre'.
'lyre'+'bird'='LYREBIRD'
'is a fine-feathered' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lyrebird that I've seen before include "Very long-tailed flier" , "Australian long-tailed avian" , "Australian native" , "Australian pheasant" , "mimic" .)